Ultrasonic inspection is one of the most important nondestructive techniques for inspecting materials and structures. Conventional ultrasonic inspection suffers from two important limitations: first, there is need of contact between the transducer and the inspected part or need of coupling fluid bath or fluid column (such as water) to transmit ultrasound and secondly the transducer should be properly oriented with respect to the surface when single side inspection is performed (operation in reflection or pulse echo mode). Thus, inspection of samples at elevated temperature or complex geometry is difficult. These limitations are circumvented by laser-ultrasonics, an ultrasonic inspection technique which uses lasers to generate and detect ultrasound. For generation, a high power short pulse laser is generally used and the ultrasonic waves are produced by the surface stresses induced by the heat source deposited by laser absorption or by the recoil effect following surface ablation. For detection, a continuous wave or long pulse laser is used in association with an interferometer which demodulates the frequency shift produced by the ultrasonic surface motion and gives a signal representative of this motion.
For industrial inspection, in contrast to investigations in a laboratory, interferometers based on velocity interferometry are preferred, as explained in a review paper "Optical Detection of Ultrasound" by J-P Monchalin published in the IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, September 1986, pp. 485-499. In U.S. Pat. No. 4,659,224 Apr. 21, 1987, the present inventor teaches how a confocal Fabry-Perot interferometer can be advantageously used as interferometric receiver for ultrasound. Such a system permits to receive many optical speckles from a large spot on the surface of the sample and its detection bandwidth can be optimized by proper choice of mirror reflectivities. In this system the laser frequency is tuned to the slope of the Fabry-Perot transmission peak.
In his copending patent application U.S. Ser. No. 07/310,380 filed on Feb. 15, 1989 now U.S. Pat. No. 4,966,459, now the present inventor describes how a confocal Fabry-Perot interferometer can be used in a different way to provide broadband detection capability. The confocal Fabry-Perot is used in this case to strip the incoming light from the sample from its optical sidebands produced by the ultrasonic surface motion or transient motion. This scheme consists in using a Fabry-Perot in the reflection mode and not in the transmission mode as described in the above U.S. patent. The interference effect occurs at the front mirror, between the light directly reflected by this mirror and light stripped from its sidebands reflected by the optical confocal cavity.
For both of these detection schemes which use a confocal Fabry-Perot (or a multiple-wave interferometer of the confocal Fabry-Perot type), the detectivity and the signal-to-noise ratio improve for higher laser power when the noise in the signal is caused by the fundamental quantum nature of light (quantum noise or photon noise or shot noise). In this case, the noise and the signal-to-noise ratio vary as the square root of the power received by the detector. In practice, lasers have fluctuating intensities, so when the power received by the detector is increased, either because the surface has a better reflectivity or by increasing the laser power (such as sending it through an optical amplifier), a level is reached where the noise essentially originates from the laser intensity fluctuations. When this level is reached it becomes useless to increase further the received power, since the signal-to-noise ratio (and consequently the minimum detectable ultrasonic or transient surface motion) becomes independent of the received power. This limitation also occurs when the light reflected by the surface through the collecting aperture of the system is constantly varying, such as in the case of a liquid agitated by waves and ripples at its surface. The present invention overcomes these difficulties.